Figure 2. Overview of cranial placode development.

Placode development is a multi-step process which first starts with the induction of non-neural (blue) and neural (green) ectoderm from naïve ectoderm. The neural border (not shown) is formed in between neural and non-neural ectoderm, which gives rise to the pre-placodal region (orange) and neural crest (red). The pre-placodal region develops exclusively in the head region as a U-shaped domain surrounding the anterior neural plate and is absent from the trunk region. Neural crest is absent from the anterior-most part of the head and develops in the posterior part of head and throughout the trunk region. Some of the specific genes of each domain are also shown and color coded in accordance to their expression zone. Different cranial placodes are induced from the pre-placodal domain in response to local molecular signaling and specific transcription factors. The diagram represents a simplified and consensus view of a vertebrate embryo, but is closest to amniote embryos such as birds or humans that develop and gastrulate as a disc. Although the precise stages vary considerably in different vertebrate classes, the diagram shows an embryo prior to the onset of gastrulation (“blastula”), after initiation of gastrulation (“early gastrula”), after induction of the neural plate but before the formation of clear neural folds (“early neurula”) and after closure of the neural tube (“neurula”).